Field
The present disclosure generally relates to multi-chip modules (MCMs) and techniques for fabricating MCMs. More specifically, the present disclosure relates to an MCM that includes a photonic chip, an interposer and an optical gain chip.
Related Art
Optical interconnects based on silicon-photonics technology have the potential to outperform electrical interconnects in terms of bandwidth, component density, energy efficiency, latency, and physical reach. Consequently, optical interconnects are a promising solution to alleviate inter-chip and intra-chip communication bottlenecks in high-performance computing systems.
While tremendous progress has been made in developing silicon-on-insulator (SOI) circuits, light sources on silicon remain a substantial technological challenge. In particular, in order to make an efficient laser source on silicon, an efficient optical gain medium is needed. However, because of its indirect bandgap structure, silicon is usually a poor material for light emission. Although there are ongoing efforts to enhance the light-emitting efficiency and optical gain in silicon, an electrically pumped room-temperature continuous-wave (CW) laser in silicon remains elusive. Similarly, while there have been exciting developments in the use of germanium directly grown on silicon as the optical gain medium, high-tensile strain and high doping are typically needed to make germanium have a direct bandgap, which usually results in low laser efficiency. Furthermore, epitaxial growth of III/V compound semiconductors on silicon is often difficult because of the large lattice and thermal mismatches between silicon and the III/V compound semiconductors, which also limits laser efficiency and reliability. Thus, because of a number of material, process and device physics issues, these areas remain research topics.
An alternative near-term approach for building lasers on or using silicon is hybrid wafer integration of III/V compound semiconductors with silicon. For example, evanescent-coupled hybrid lasers have been successfully demonstrated by wafer bonding of indium-phosphide active structures to silicon via either oxide-to-oxide fusion bonding or polymer-enabled benzocyclobutene bonding. However, because of taper loss, carrier-injection efficiency and thermal impedance, these hybrid lasers typically have relatively low optical-waveguide-coupled overall efficiency. Furthermore, wafer-bonding approaches usually only work for direct bonding of III/V compound semiconductors on a silicon wafer with passive circuits. In addition, III/V compound-semiconductor wafers and SOI wafers often do not have compatible sizes. In particular, a III/V compound-semiconductor wafer size is usually limited to 150 mm, while typical SOI photonic wafer diameters are 200 mm and 300 mm. Consequently, integrating hybrid laser sources with other active silicon devices (which may include multiple layers of metal interconnects and interlayer dielectrics) remains a challenge.
Edge-to-edge butt-coupling of a III/V compound-semiconductor optical gain medium with silicon optical waveguides is a common hybrid integration approach. It allows the high electrical injection efficiency and low thermal impedance of conventional III/V compound-semiconductor lasers to be maintained. Moreover, using this approach, both the III/V compound-semiconductor optical gain media and the SOI circuits can be independently optimized for performance, and can be independently fabricated. External cavity (EC) lasers using this hybrid integration technique have been successfully demonstrated with high optical-waveguide-coupled overall efficiencies. However, because of the optical-mode mismatch between the III/V compound-semiconductor and the silicon optical waveguides, special mode size converters on either or both sides of the III/V compound semiconductor and silicon are often needed. In addition, in order to obtain efficient optical coupling, accurate alignment of the optical waveguides in the III/V compound semiconductor and the silicon with sub-micron alignment tolerances is typically needed. Addressing these problems can decrease the yield and increase the cost of these hybrid lasers.
Hence, what is needed is a multi-chip module (MCM) and a technique for fabricating an MCM without the problems described above.